An Exhaust Gas Recirculation(EGR)system reduces unwanted emissions resulting from the combustion process in an internal combustion engine. When combustion occurs in an environment with an excess of oxygen, peak temperatures in a combustion chamber increases leading to the formation of NOx. The EGR system introduces exhaust gas having a low oxygen concentration into an inlet manifold of the internal combustion engine to lower the concentration of oxygen. By reducing the oxygen concentration, fuel burns slower and reduces peak temperatures in the combustion chamber. Also, the recirculated exhaust gas absorbs some of the heat released during combustion.
Current EGR systems are generally used when an exhaust manifold pressure is greater than the pressure in the inlet manifold. In a pressure charged engine, including turbo charging and super charging as examples, the pressure in the inlet manifold typically increases as the engine load increases. As the pressure in the exhaust manifold approaches the pressure in the inlet manifold, the exhaust gas recirculation flow in a fixed diameter orifice or duct between the inlet manifold and the exhaust manifold decreases. Higher engine speeds and engine loads also generally result in an increase in NOx emissions. Conventional EGR systems provide little, if any, exhaust gas recirculation during times when the engine is producing the most NOx, because the low pressure differential between the exhaust manifold and the inlet manifold prevents sufficient exhaust gas from entering the inlet manifold.
Large truck engines using EGR systems provide an example where no exhaust recirculation may be available at high engine speed and high load. In these applications, the pressure of the air in the inlet manifold can become greater than the average pressure of exhaust gas in the exhaust manifold thereby preventing the use of conventional EGR systems at high loads and speeds.
One way of maintaining a recirculation flow of exhaust gas in a high load, high speed condition is by maintaining a high back pressure in the exhaust manifold. A high back pressure increases the pumping work associated with expelling exhaust gas. The increased pumping work results in a less efficient engine most notably seen by an increase in brake specific fuel consumption.
Instead of maintaining a high back pressure, some control systems use a venturi nozzle to reduce the pressure at a predetermined location in an inlet conduit. An example of this is shown in Henderson U.S. Pat. No. 5,611,203 issued Mar. 18, 1997. Inlet air is throttled through the venturi nozzle where the pressure is reduced in a throat area thereby increasing the pressure differential between the exhaust manifold and the inlet air conduit. An ejector pump is located in an exhaust gas recirculation conduit to further increase the pressure differential and resulting flow of recirculated gas. To achieve any meaningful control, these systems require additional sensors and controls to account for the widely varying pressure differentials between the pressure in the throat and the exhaust manifold.
Another way of managing exhaust gas recirculation at high load, high speed conditions uses a feedback control method that varies an EGR flow control valve based on mass flow rates or other engine parameters. Such feedback control methods require extensive testing and refinement to ensure robustness. A valve located between the exhaust manifold and the inlet manifold provides a variable volume of recirculated exhaust gas during specific operating conditions where the exhaust manifold average pressure is greater than the inlet manifold pressure.
Current methods for controlling exhaust gas recirculation systems may meet emissions standards, but they incur a number of penalties. EGR systems effectively reduce the power density for each charge by limiting the amount of oxygen available for combustion. By limiting power, EGR systems effectively decrease overall performance including increasing break specific fuel consumption. NOx formation increases dramatically as the engine enters the high load operating range, but current EGR systems are not able to operate in high load range conditions. Instead of reducing NOx in a limited range where its production is greatest, current EGR systems operate over the entire engine range excluding the high load condition. The performance penalty of the current EGR systems propagates throughout a larger engine operating range. The present invention is directed at overcoming one or more of the problems set forth above.